The present invention relates to a method for the manufacture of a cooled piston for use in internal combustion engines. Particularly, the invention deals with a method to manufacture the upper part, or head of a two-piece, or articulated, piston provided with improved cooling means.
The main components, i.e. the head and the skirt, of articulated pistons are independent members. The head, composed of the top and the pin boss portions, is generally made of forged or cast steel, and the skirt is made of a lighter material, generally aluminum or an aluminum alloy. This type of piston is being adopted for diesel engines, especially last generation diesel engines, where combustion pressures and temperatures in the engine are very high. Owing to these conditions, which are extremely adverse to the piston head, it has been a common practice to provide the piston crown with a cooling chamber for the circulation of oil to remove part of the heat from at least two regions very susceptible to the influence of elevated temperatures: the ring band and the combustion bowl rim.
One traditional cooling chamber construction consists of providing a circumferential groove at the piston crown between the combustion bowl and the ring band, and a circumferential trough, or tray, on the skirt upper portion, whereby with the piston assembled the groove and the tray define a semi-open cooling chamber for the circulation of cooling oil.
While suitable for most applications, this type of chamber is not entirely satisfactory when a higher cooling efficiency is required. One disadvantage is a poor cooling of the undercrown, i.e. the region underneath the combustion bowl, which is not reached by the chamber cooling action. Another disadvantage arises from the construction itself of the semi-open chamber: part of the cooling oil flows from the tray on the skirt upper portion thereby reducing the quantity of oil in the chamber and, accordingly decreasing the cooling efficiency on the ring band and combustion bowl wall regions.